Cubic Polynomial Generator, Interpolator and Plotter

This script generates a cubic polynomial from four points. The x coordinates x1 through x4 must all be different. The script will also interpolate (or even extrapolate!) to give a y value for any x coordinate, x5. Just for the sake of example, the page automatically loads width the results from the four points (-1,-1), (2,2), (3,1) and (4,2) with an interpolation to x=2.5.